1. Field of the Invention
The invention relates to digital subscriber line (xDSL) communications and more particularly to identifying crosstalk in an xDSL system.
2. Background Information
Crosstalk in a communications system is the undesired effect or interference in a channel or circuit caused by another circuit or channel. Crosstalk interference by the adjacent pairs is one of the major performance limiting factors of xDSL systems.
FIG. 1 illustrates a typical communications system such as an xDSL system. Central office equipment (CO) 102 communicates to at least one consumer premises equipment (CPE) 104 (and in this example CPE 106 as well). Data transmitted towards the CPE shown by lines 108 and 112 is referred to as downstream. Data transmitted towards the CO shown by lines 110 and 114 is referred to as upstream. For a given line, crosstalk typically occurs when the interfering disturber is in close proximity to the line, which most likely occurs near either the CO or the CPE, because that is where there is a likelihood of aggregating lines.
FIG. 2 illustrates the various types of crosstalk typically experienced in a communications system. For simplicity, two CO's and two CPE's are shown. CO 202 is in communications with CPE 204 and CO 206 is in communications with CPE 208. For the sake of example, the crosstalk from CO 206 and CPE 208 to either CO 202 or CPE 204 is described. The term “far-end” refers to when the interference is away from the receiving side and the term “near-end” refers to when the interference is close to the receiving side. For example, interference shown by arrow 212 illustrates noise generated by CO 206 coupled into the downstream communications and received by CPE 204. The term “victim” is applied to the line or the circuit being examined for crosstalk, and the term “disturber” is applied to the source of the crosstalk. Since the noise is generated away from the receiving side, this is referred to as downstream far-end crosstalk (FEXT). Likewise, interference shown by arrow 214 illustrates upstream near-end crosstalk (NEXT). Interference shown by arrow 216 illustrates upstream FEXT, and interference shown by arrow 218 illustrates downstream NEXT.
In xDSL communications, the communications bands are divided channels known as bins or tones. For example, the asymmetric digital subscriber line (ADSL) bins are 4.3125 kHz wide. Typically, the upstream communications uses fewer bins (for example, 26 bins spanning from 25.875 kHz to 138 kHz) making them more susceptible to interference.
Furthermore, FEXT, in general, is typically less severe than NEXT as it gets attenuated by the loop. In ADSL, the available lower bandwidth for the upstream channel might be severely affected by the low frequency components of NEXT generated by sources such as single line high rate digital subscriber line (SHDSL). Also, the number of NEXT sources or potential disturbers at the CO side is higher due to large density mix of services at the CO. Further, self-NEXT can become the dominant NEXT disturber at long distances because other services do not extend so far. Therefore, upstream NEXT crosstalk is the most crucial interference to address for ADSL systems
In the past, the identification of noise and interference required loop qualification and line testing that had to be performed by deploying technicians to both ends of a connection and having them take measurements in a coordinated manner using special test equipment. Recently, xDSL standards such as ADSL2, ADSL2+, or very high speed digital subscriber line 2 (VDSL2) have recognized that xDSL modems are capable of transmitting and processing the necessary test signals, and of controlling the collection and exchange of these measurements from one end of the connection, typically the CO end. Within these standards, the capability is referred to as Dual Ended Line Testing (DELT).
A DELT session can be requested by either the CO or the CPE, and is often requested as part of the discovery phase of the initialization procedure. The difficulty is that the results, such as Quiet Line Noise (QLN) measurements, from the DELT session provide raw measurements, but such measurements in raw form do not provide meaningful information to a service provider. One approach applied in the past is to find a maximum correlation with a “basis or a representative set” of measured crosstalk coupling functions. This basis is supposed to be representative of the crosstalk disturber. However, such an approach does not consider the fact that the basis set is shaped by the effects of the front-end, such as filter effects. Also, in a real world situation, in addition to the crosstalk disturber (which is a wideband disturber), a narrowband disturber may generate radio frequency interference (RFI) which can arise from radio transmissions such as amplitude modulations (AM) radio transmitters and amateur radio transmitters like HAM radios and operate at frequencies that fall into the frequency bands of xDSL. This situation is not accounted for by the correlation approach just described. Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies.